The global demand for energy continues to rise while reserves of conventional petroleum (e.g. oil, gas, and natural gas liquids) are in decline. This has led to increased focus and research into unconventional fuel resources (e.g. heavy oil, oil sands, oil shale) and other non-fossil sources of energy (e.g. lignocellulosic materials).
A significant amount of research in the field of “alternative” energy production has focussed on the generation of biofuels from lignocellulosic matter. This technology raises the prospect of a shift to an abundant and renewable feedstock for energy production as an alternative to the depleting reserves of hydrocarbon-based raw materials. The enrichment of low energy density fossil fuels (e.g. lignite, peat and oil shale) into high energy fuel products also represents an attractive alternative given the relative abundance of those resources.
In particular, the thermochemical conversion of biomass and other complex organic matter into biofuels and chemicals based on hydrothermal reactions has shown significant promise. In general, it is desirable that such methods are continuous or at least semi-continuous in nature which may lead to improved product characteristics and/or improved process economics in comparison to batch processes. Process economics are also more favourable when increased concentrations of organic matter are used in the thermochemical conversion steps, because the amount of water or other solvent that must be heated to elevated temperatures is less. However, when high concentrations of organic matter are converted at elevated temperature and pressure the main products are frequently viscous solutions. A common problem in such situations is a partial de-solubilisation of organic and incidental inorganic matter, leading to deposition on apparatus surfaces, otherwise known as “sealing”. Additionally, when water is used as the primary depolymerisation agent swelling of organic matter can occur restricting the concentration that can be used. The high levels of energy needed to raise and maintain water at reaction temperature can also result in charring on the inside of reactor vessel walls. With prolonged operation such deposits can have an adverse effect on the process, necessitating time-consuming and costly descaling operations in order to restore process performance. Furthermore, at high concentrations of organic matter, the present inventors have observed that a pressure differential (i.e. a pressure gradient) develops along the length of tube reactors under continuous flow operations which is detrimental to process efficiency.
A need exists for improved methods capable of reducing or avoiding problems such as scaling, charring and/or the development of pressure gradients across reactors during the thermochemical conversion of organic matter into bio-products.